Method and apparatus for analyzing substance by mass spectrometry



Feb. 13, 1951 R. w. LONG METHOD AND APPARATUS FOR ANALYZING SUBSTANCE BYMASS SPECTROMETRY 2 Sheets-Sheet 1 Filed July 18, 1947 zucmooum 0? Feb.13, 1951 R. w. LONG 2,541,656 METHOD AND APPARATUS FOR ANALYZINGSUBSTANCE BY MASS SPECTROMETRY Filed July 18, 1947 2 Sheets-Sheet 2 'I FF on BEZI: I

T \J T 67 5 g COLLECTOR i ELECTRODE i 50 ELECTRODE l In 'PUSH- PULL A.G.AMPLIFIER qu ymvmvrox. I A @154 Patented Feb. 13, 1951 UNITED STATESPATENT OFFICE METHOD AND APPARATUS FOR ANALYZING SUBSTANCE BY MASSSPECTROMETRY Robert Warren Long, Winter Park, Fla., assignor,

by mesne assignments, to Standard Oil Development Company, Elizabeth, N.J., a corporation of Delaware Application July 18, 1947, Serial No.761,789

'7 Claims.

wherein the gaseous material is bombarded with low energy electrons suchthat a small portion of the molecules are converted into ions ofdifferent masses. Each ion usually possessesa unit of positiveelectricity. The ions are forced into an accelerating field where theyacquire equal kinetic energies and then pass into an analyzing portionof the mass spectrometer which usually comprises a curved conduit withinanexternal magnetic field.

The ions moving at a high velocity through the analyzer portion of themass spectrometer constitute an electric current and a magnetic field isproduced simultaneously by the moving ions. An external, crossedmagnetic field acts upon the magnetic field caused by the moving ionsand causes the ions to be deflected in-the curved conduit. The magnitudeof the magnetic field and/or the magnitude of the accelerating voltagecreating the accelerating field may be controlled such that ions of onlyone mass pass through an exit slit of the curved conduit of the analyzerand tothe ion collecting means. The ions'impinge on a Faraday cage'orother collecting means which is grounded through a resistance and theneutralization of the charge carried by the ions causes a current toflow through the resistance and set up a potential drop. The potentialdrop is conventionally amplified in a D. C. amplifier and measured bymeans of a sensitive galvanometer or recorder. The current set up byneutralization of the ions impinging on the collecting plate isproportional to the number of ions striking the plate per unit of time.By proper manipulation of the external magnetic field and/or theaccelerating electric field strengths, ions representing components ofdifferent molecular weights or masses can be caused to pass to thecollector plate in sequence in order to obtain a record or a massspectrum representing the composition of the substance.

In the conventional type of mass spectrometer, positive ions produced atthe ion source are usually measured. Because of a small potentialgradient maintained in the ion source, positive ions are caused topassinto the ion accelerating field and are eventually passed throughthe analyzer portion of the mass spectrometer, whereas negative ionsformed at the ion source are repelled and are thus prevented frompassing to the accelerating field. Thus, the ions passing through theanalyzer form a non-pulsating or steady positive ion beam whicheventually impingesron the collector plate as mentioned above and areneutralized by a fiow of electrons from ground. The electron flow isdirectly equivalent to the positive ion charge flow to the collectorplate, and the I (current) R (resistance) =E (voltage) difference acrossthe ends of the grounded resistor is directly related to the currentthrough the resistor. As previously mentioned, this current is relatedto the number of ions collected per unit time. Hence the variations ofthe number of ions collected per unit time are converted into signalsrepresented by voltage changes which are impressed on an electronic tubedesigned to amplify the magnitude of the original current to a magnitudeamenable to standard measuring devices or recorders.

It is thus seen that direct currents set up by the neutralization ofthedirect ositive ion beam on the collector plate are amplified andmeasured in the conventional equipment. Direct current amplification ofthe voltage change, however, hascertain drawbacks. It is known, forexample, that A. C. amplifiers are inherently more stable than D. C.amplifiers. Direct current amplifiers also are subject to what is knownas zero drift resulting from the fact that aging of filament, minutechanges of potentials within the electronic tubes, and similardisturbances upset the balance of conditions previously establishedcausing a new set of operating parameters to exist. These new parametersobviously produce a shift of the apparent balance which is interpretedas a zero drift. A. C. amplifiers respond only to signals of periodiccharacter and have no output for no input signal; hence, in thequiescent period, no output is obtained which means that no shift ofbalance can appear. Direct current amplifiers are also known to haveless usable sensitivity and to respond more slowly to voltagefluctuations than do A. C. amplifiers. It would, therefore, be extremelydesirable to have available a means for using A. C. amplification of theion beam currents in order to eliminate the aforementioned difficulties.The present invention is directed to a novel means of forming apulsating ion beam 3 signal amenable to amplification in an A. C.amplifier.

It is the principal object of my invention to provide a means forimproving the operation of a mass spectrometer.

It is another object of the present invention to provide a magneticmeans'for modulating a nonpulsating ion beam such that a pulsating ionbeam signal is formed.

It is a further object of the present invention to improve the stabilityand sensitivity/of amass spectrometer with respect to the amplificationand recording of signals produced subsequent to the collection of ionbeams of "various'masses.'

Briefly, the present invention involves born-- barding a gasiformmaterial with an electron beam such that ions of various masses areformed, passing the ions through an analyzer portion of a massspectrometer to segregate an posed on the steady ion beam bythe'magnetic The present inventi'outakes' advantage of the fact thatthe'steady ion beam comprising ions of any one mass leaving the analyzerportion of'the mass spectrometer"constitutes an electric current whichproduces a magnetic field; By subjecting" the magnetic'field produced bythe movingions to an auxiliary magnetic field periodic in character andof one polarity or'to a magneticfield of constantly changing polarity,it is possible to defiect alternately the ion from the collectingportion of a mass spec trometer such that only a portion of the ionsreach the collecting means and, consequently, only a portion of the ionsare periodically neutralized by a flow of current from the groundedresistance attached to the collecting means; The periodic flow ofneutralizing current results in the formation of a pulsating signalwhich is amenable to amplification in an AC. amplifier.

Likewise, by providing the collecting portion of a mass spectrometerwith a plurality of collecting means electrically connected throughsepa-' rate channels to a push-pull A.'C. amplifier, the ion beam may bedeflected frombnc collecting meansto another ialternately by means of amagnetic field of constantly changingpolar ity which results inproducing two pulsating signals which may be amplifiedfand subsequentlycombined for use as a means of indicating the relative abundance ofions'of any one mass' in the original ion beam. The practice of thepresent invention is usefuI'in that no matter what types of collectingmeans are employed, the principle is employed in which a magnetic field,

periodic in character, alters the course of the non-pulsating ion beamto produce a pulsating signal which is a function of the number of ionscollected per unit time, The pulsating'signal is more easily controlledand amplified than nonpulsating signals produced in conventional massspectrometers.

Several embodiments of the device of the present invention will now bedescribed in detail in conjunction with the accompanying drawing inwhich Figure 1 is an elevation, partly in section, with conventionalelectric units comprising a side view of theassembly along withconventional symbols,

Figure 2 is a side view of the collecting portion of the massspectrometer assembly shown in Figure 1,

Figure 3 shows an end view of the collecting portion of themass's'pectrometer, shown in Figure 1,

Figure 4 shows a side view of another embodiment of the-collectionportion of a mass spectrometer,

Figure 5 shows the geometrical relationship between 'the variouselements of the collection portion of the mass spectrometer illustratedin Figure 2, and

Figure 6 shows the geometrical relationship of the various elements ofthe collection portion of the mass spectrometer illustrated in Figure 4.

Turning now to -Figure 1, the numeral H designates the ionizationchamber of a mass spectrometer, numeral I 2 designates the analyzerportion of the mass spectrometer and numeral 13 designates thecollection portion of the mass spectrometer tube. The particular tubeshown is of the type irequently used in the analysis of gaseousmixtures; My invention is not limited, however, to any specifictype'of-mass spectrometer tube but is applicable to analysis of gasiformmaterial in the horizontal, 60,

and other types of mass pectrjometer-tubes.

The mass spectrometer tube may be constructed of glass and is connectedto a sample system It containingdr'awout electrode l5. The tube also isconnected to a vacuum pump system It which maintains the entiresystem-under an extremely low pressuresuch as from 10 to- 10-millimeters of mercury, absolute pressure, and withdraws from the tubeunreacted portions of the gasiform material introduced therein. Theionization chamber 1] of the mass spectrometer tube also containsfilament" ll, made of tungsten or other'such material, which isconnected to an electronically regulated power supply, not

shown. Also contained in -the ionization chamber are electrodes l 8 andIQcQntaining'aIigned I slits 20 and 2i,respectively, andelectroncollector electrode 22. Electrodes 23 and '2 placed directlyunder drawout electrode 15 contain aligned slits 25 and 26,respectively, these two electrodes conveniently being termed the ionsource of the mass s ectrometer; H

The analyzerportion I2 of the mass spectrometer tube'containstwoelectrodes 27 and 28 placed at opposite ends of the curved conduit andcontaining slits 29 and 30, respectively. The analyzer electrodes areconnectedfthr-ough an internal non-magnetic shield 3|. The curvedportion of the mass spectrometer is surrounded by an external crossedmagnetic field-BZ, which is produced by a suitable electr o magnet 33,the

magnitude of which is -cont rolled by a power supply, not shown. The ioncollection portion of the mass spectrometertube 13 comprises thedecelerating field made upof elec'trode 34 containing slit 35 andshielding electrode 36 containing slit 3'! and'the collecting electrode'38. Electrode 3B is connected through lead to grounded resistance 40and A. C. amplifier 4|. The A. C. amplifier is connected:through'suitable leads represented b numeral 42 to 'a rectifier system43 which, in turn, is electrically con-' nected through lead 44 to agalvanometer or recorder system 45.

The numerals 4B and itv represent the poles of an external magnet forproducing a transverse magnetic field for the collection portion of themass spectrometer tube. This magnetic field, which will be referred toby the numeral 46, will be described in more detail hereinafter. InFigure 1, it is noted that ion source electrodes 23 and 24 are connectedthrough leads Al, 48 and 39 to ground and that analyzer electrodes 21and 28 are connected through leads 50 and 5| to a source of highpotential at junction 52. Drawout electrode i5 is connected to a sourceof potential by means, not shown, the magnitude of which is slightlyhigher than the magnitude of the potential impressed on electrodes 23and 24.

Figure 2 illustrates a side view of the collection portion of the massspectrometer tube, showing electrodes 34 and 36 and collector electrode38. The poles of the electromagnet represented by the numerals 46 and46' are connected to a suitable electr-omagnet 53.

Figure 3 shows the end View of the collecting portion of the massspectrometer, the numeral !3 designating the mass spectrometer tubeitself, the numerals it and 46 representing the poles of the magnet andthe numeral 53 showing the electromagnet supplying energy to the poles4E and 4.6. Current for magnet 53 is supplied through lead 54electrically connected to oscillator 55 and lead 55 containing resistor51 which is electrically connected through lead 58 to oscillator 55.Oscillator 55 determines the frequency of the changes of polarity inelectromagnet 53. A suitable Voltage regulator may also be incorporatedin the system if desired to regulate the current for the electromagnet.

Figure 4 shows a side View of another embodiment of the collectingportion of the mass spectrometer tube, partl in section, which containsanalyzer electrode 28, decelerating electrode 34, shielding electrode 36and two collecting electrodes 59 and 69, respectively. Electrode 59 iselectrically connected through lead 6! to grounded resistor 52 and toone channel of pushpull A. C. amplifier 63. Electrode 55 is likewiseconnected through lead (54 to grounded resistor 65 and to the otherchannel of push-pull A. C. amplifier 63.

Returning now to Figure 1, the operation of the mass spectrometer willbe described in more detail. A small amount of gasiform substance to beanalyzed is introduced from drawout electrode 55 into the dissociationregion existing between electrodes l8 and l9. An electron beam E58originating from heated filament il flows in a path perpendicular to theilow of the gas from electrode l5, this electron beam being maintainedin a horizontal fiat ribbon through the dissociation region by means ofan externally applied magnetic field, not shown, such that electronsimpinge on electron collector 22. A. portion of gaseous moleculesentering the dissociation region are bombarded by the electrons and formions of various masses. A small potential difference of proper polarityis maintained between drawout electrode i5 and electrode 23 such thations (either positive or negative) are slowly moved t wards acceleratingfield entrance slit 25 in elec trode 23. The ions then pass, throughfocusing electrode slit 2E and are accelerated in the field existingbetween electrodes 2 and 274 due to the large potential differenceexisting between these two electrodes.

A portion or the ions then passes through i alyzer entrance slit ZQan-dinto magnetic field 32. By controlling the magnitude of magnetic field32 and/or the strength of the accelerating field existing betweenelectrodes 2 and 23, ions of any one selected mass may be made to focuson analyzer exit slit 3d, the remainder of the ions impinge on shield 3iwhereon they are neutralized and pumped out of the system by means ofvacuum pump 26. The ions focused on exit slit 30 designated by thenumeral 575' constitute ions of any one mass in a steady or direct beam.The ion beam 61 then enters the decelerating portion of the collectorend of the mass spectrometer tube, the decelerating field existingbetween electrode 28 and grounded electrodes 3 and 35. Since electrodes34 and 35 are maintained at substantially the same potential aselectrodes 23 and 24, the ions are slowed down in velocity to the sameextent that they were speeded up in accelerating field existing betweenelectrodes 24 and 2?. The ion beam reaching electrode 38 is, therefore,slowed down to a few volts energy approximately being equival nt to thepotential difierence between drawout electrode and electrodes 23 and24'.

Ion beam 6'! is subjected to magnetic field of periodically changingpolarity such that only a portion of the ions impinge on collector plate38. The method of ieflecting the ion" beam ofi and on the plate 38 willbe described in more detail hereinafter. The portion of ions impingingon collector plate 38 is neutralized by a fiow of current received fromground which flows through resistor ii? to the collector plate. Theneutralization of the charge carried by the ions set up a potential dropwhich is applied to A. C. amplifier 4! containing amplifier tubes suchas 68. The signal formed at collector plate 38 and resistor :38 isperiodic in character, A. C. amplifier 3!, which may be tuned to thedeflecting frequency imposed on ion bear-1 ii'E, will amplify thesignal, rectifier 33 will change amplified A. C. signal into a D. C.signal which may be recorded by conventional methods in recorder d5.

Turning now to Figure 2, it is seen that ion beam 67 passing in themagnetic field 3? is alternately deflected from one side of collectorplate 38 to the other. Magnetic field of constantly changing polarityhas a strength just sufficient to cause ion beam ill to be deflectedfrom either side of the collector plate, the portion of the ion beamsweeping across the collector plate being neutralized by a flow ofcurrent through the grounded resistance as hereinbefore described andthe portion of the ions deflected off or" bh collector plate impingingon a suitable internal shield Within collecting means !3 and beingneutralized and pumped out of the system.

Turning now to Figure 3, the method of controlling the polarity andstrength or" magnetic field 46 is shown. Voltage is supplied tooscillator 55 through leads 59 and Ed, oscillator 55 controlling thefrequency of the polarity change of the magnetic field. Oscillator isdirectly connected through lead 54 to eleetrornagnet 53 and indirectlythrough resistor 53 and lead 55. By changing resistor 5?, it is possibleto change the strength of the magnetic field of poles and 46.

The operation of the collection portion of a mass spectrometer shown inFigure 4 is similar to that previously described. Collecting elec trodes59 and 66 having equal areas are separated by a very small space and thestrength or 7 magnetic field 46 is adjusted such that ionbeams 61 willbe alternately deflected from one collector plate to the other. Duringany portion of the time that the ion beam is on one or the othercollector plate, the ions are neutralized and. a potential drop is setup with the grounded resistance connected to each collector plate.During the portion of the time that the ion beam. is passing from onecollector plate to the other, it impinges on an internal shield, notshown, which may suitably be placed behind the collector plates and theions are neutralized and pumped out of the system. Therefore, twopulsating signals are set up in leads SI and 64, each being carried toits respective channel in a conventional push-pull A. C. amplifier E3.The-separate signals may subsequently be combined, rectified, andrecorded.

Turning now to Figure 5, the mathematical derivation of the magneticfield strengths of alternating polarity required to deflect an ion beamfrom a single electrode will be discussed in detail. When the axis ofthe external coil mag netic system is perpendicular to the axis of theion collector system, a magnetic field perpendicular to the path of ionbeam 6'! may be determined by the following equation:

Equation 1 2 H ev where H=magnetic field strength in oersteds, e=chargein E. M. U. (electromagnetic units) on m mass having v=velocity andr=radius of circle of resulting path.

It is seen that for any one mass, e, v, and m are constant; therefore,the curvature of the path,

is proportional to H, the magnetic field strength. Equation 1 may besuitably written in terms of voltages by use of the following equation:

Equation 2 V010 mo where V=accelerating voltage, e=charge in E. M. U. onm=mass of particle, where o=velocity resulting therefrom.

By combining Equations 1 and 2, the following is obtained:

Equation 3 H er 1 -=2 1 l and when mn=mass hydrogen and M=mass number,Equation 3 becomes:

Equation 4 H r =2MV-10 It is seen that the magnitude of H may besuitably adjusted to control the extent to which ions of any one massare deflected. Use may be madeof Equation 4 to determine the minimum Hnecessary to deflect ions having a mass 925 away from a collectorelectrode having a diameter of 0.6 centimeter. In Figure 5, it is seenthat a magnetic field H has been imposed on ion beam 6': causing the ionbeam to have a radius of cur vaturer and to be deflected the distance accausng it to miss the collector electrode 38. D-repreand. since a: isapproximately equal to which, when substituted into Equation 4, gives:

EqaationS zflvaor 1 e/m D (2 Whr H is the minimum magnetic fieldstrength to cause the ion beam to be deflected off of the collectorplate. When M :100 mass number, V=l9 volts, e/mn=0.96 10 E. M. U./gram,D=2 centimeters, and

%=O.l centimeter the minimum H as determined from Equation 5 is:

HEZIO oersteds If a smaller collector plate is used than is shown above,the H may be decreased. Specifically, for a collector plate having aradius of 0.7 mm., H oersteds. Obviously, for ions hav ing masses belowthe magnetic field'strength requirements will be lower than those shownabove.

Turning now to Figure 6, a difierent situation obtained to thatpreviously described in conjunction with Figure 5; that is, the ion beamis deflected from one of a plurality of collector plates to another. Thedistance d in this case is the space between electrodes 59 and ac and a:is the distance the magnetic field H deflects the ion beam from itsusual path to cause impingement on one of the electrodes' It is readilyseen that if d, one obtains the minimum distance the ion beam musttravel to contact one of the electrodes. By employing the reasoning usedin connection with Figure 5, and assuming that d=0.02 cm. with othervalues the same as previously used substituted in Equation 5, HE22oersteds or less.

It is seen from the foregoing discussion that the magnetic fieldsrequired to produce deflection of ions having masses as high as 100 areeasily produced with currents available from conventional electronicapparatus. Since the frequency of the deflecting magnetic field imposedon the ion beam controls the wave form of the resulting signal produced,it is possible to tune the particular type of A. C. amplifier used tothe interruption frequency to amplify the pulsating signal. The methodof the present invention is effective in that the magnitude of thecollecting currents are very easily controlled. Suitable magnetic fieldstrengths may be employed which will cause the ion beam to avoid thecollector plate over any portion of the modulation cycle; that is the.pulse to signal ratio may be varied from pulse time cycle time Thesystem illustrated in Figure 4 is more effective than the systemillustrated in Figures 1 and 2 in that a greater portion of the energyof the ion beam in the system is utilized than in the latter system.This is because a smaller quantity of the ion beam is deflected from thecollector plates when using two or more collector plates than is thecase when using only one collector plate.

The magnetic field employed to deflect the ion beam from the collectorplate, when employing only one collector plate, may be of any onepolarity but made periodic in character by periodically interrupting thedirect current to the magnet. In other words, only one polarity of themagnet is required in which the axis of the external coil system isplaced perpendicular to the axis of the ion collector system such thatthe magnetic field is perpendicular to the path of the ions. Byemploying the magnetic field in this manner, the ion beam may bedeflected from the center portion of the collector electrode to justbeyond one end of the electrode which sets up pulsating signals amenableto amplification in an A. C. amplifier.

When employing two or more collector elec trodes, it is preferred tochange the polarity of the magnetic field while making the changeperiodic in character such that the ion beam may be deflected in twodifferent directions as previously described. Regardless of whether theperiodically varying magnetic field is of constantly changing polarityor of one polarity and periodic in character, the pole pieces or coilsproducing the field should be of such size as to cause deilection of theion beam without either affecting the focusing of the ion beam orchanging the character of the signal resulting from the collected ionbeam. The rate at which the magnetic field deflecting the ion beam isvaried will depend to a large extent on the type of A. C. amplifieremployed. Signals pulsating at a rate of from about 60 to 380 cycles persecond are generally preferred; however, the deflecting magnetic fieldmay be varied to produce signals pulsating at lower or higher rates thanthe range heretofore mentioned under certain conditions.

Another method of carrying out the deflecting system comprises employingshielding electrode 36 as the means for cutting off the deflected ionbeam. For example, in Figure 2 the slit .3! in shielding electrode 36may be made of such size that, as the ion beam is deflected in onedirection,

.the beam will contact a portion of the electrode and the ions will beneutralized. When the magnetic field changes in polarity, the ions aredeflected across the collector electrode in another direction and areinterrupted by another portion of the shielding electrode There areother methods of cutting off the deflected ion beam which may bepreferred under some circumstances. but it is to be understood that anymethod of employing a magnetic field to obtain the desired deflection ofthe ion beam is within the scope of the present invention.

the d cription of the present invention, I ha e shown [.1149 method ofcarrying out the magnetic modulation of the ion beam in which the ionsource comprises electrodes 23 and 2 and grounded and analyzerelectrodes 2'! and 2%! are at high potential while electrodes in thecollectof the magnetic field that would be required if the ion sourcewere at high potential and the ion beam passing to the collecting platehad extremely high velocities and high energies.

It is also obvious to one skilled in the art that for ions of any givenmass, a specific magnetic field strength will be required to deflect theions from a collector plate of a given size. As ions of various massesare focus-ed on the collector sys tem, it may be necessary to change thestrength of magnetic field 35 in order to obtain the desired deflectionof the ion beam. In order to accomplish a change of the collectormagnetic field strength, it may be desired to coordinate changes inresistance El shown in Figure 3 with any change imposed on the magneticfield 32 or the accelerating potential imposed at junction 52 shown inFigure 1 when various ion masses are being analyzed. It may not benecessary under some circumstances to change the strength of magneticfield it each time a different mass is focused on exit slit 33particularly if the slit cpeningtl in shielding electrode 38 is of suchsize that the ion beam may be cut off by this electrode when it isdeflected from one portion of collecting electrode 38 to another portionof this electrode. When analyzing for masses ranging from 2 to 25,approximately the same magnetic field 55 may be employed for anyparticular mass. In the range of masses from 25 to 50, the strength ofthe magnetic field 4% may be changed in order to obtain the desireddeflection of the ion beam. Similarly, for masses ranging from 50 toanother change may be made on the magnetic field 26 to obtain thedesired deflection of ion beam.

It is to be understood that the present invention is applicable to theanalysis of an ion beam comprising negative ions as well as positiveions. If it is desired to analyze for negative ions, proper changes mustb made throughout the mass spec trometer." with respect to polarity inorder that only negative ions formed by bombardment of gasiform materialwith electrons are accelerated through the analyzer portion of the massspectrometer.

The'method and apparatus described in the present invention isapplicable for testing a wide variety of solid, liquid or gaseoussubstances. The procedure is only limited in that the substance bevaporizable under the low pressure conditions existing in the massspectrometer.

The nature and objects of the present invention having been fullydescribed and illustrated, what I wish to claim as new and useful and tosecure by Letters Patent is:

1. In a mass spectrometer including a means for focusing a non-pulsatingbeam of ions having a selected mass on a collecting means, means forproducing a periodically varying transverse magnetic field followingsaid focusing means and ahead of said collecting means whereby said beamis periodically deflected from said collecting means.

2. In a mass spectrometer including means for producing a non-pulsatingbeam of ions of a particular mass, means for collecting said beam andmeans for focusing said beam on said collecting means, the improvementwhich comprises means for producing a periodically varying transversemagnetic field arranged following said focusing means and ahead of saidcollecting means whereby said non-pulsating beam is periodicallydeflected from said collecting means.

3. A mass spectrometer including a mean for forming a beam of ionshaving a selected mass which com-prises, in combination, a plurality ofion collecting means, amplifying means electrically connected to each ofsaid collecting means, means for producing a periodically varyingtransverse magnetic field arranged external to and in front of saidcollecting means whereby said beam -of .ions is periodically deflectedfrom one to another of said plurality of collecting means.

4. A mass spectrometer apparatus including means for forming ahomogeneous beam of ions corresponding to a selected mass, means forfocusing said homogeneous beam upon ion collecting means, a collectingmeans comprising a pair of ion-collecting members arranged adjacent eachother and spaced onopposite sides of the focus path of said beam, meansfor producing a periodically varying transverse magnetic field arrangedexternal to and spaced in front of said collecting'means Wherebysaidfocused beam is periodically deflected from oneto the other of said pairof collecting members, and a push-pull amplifying means having the inputterminals thereof electricallyrconnected inshunt to said collectingmembers.

5. In a method for operating a mass spectrometer wherein a beam of ionscorresponding to a selected mass is formed and is focused upon acollecting. member to produce an'electric signal, the improvement whichcomprises subjecting the focused beam of ions to a transverse fieldforce of periodically changing polarity and of such strength that saidfocused beam is periodically deflected from said collecting member, andcol- 12 lecting a portion of the resulting-deflected ion beam whereby apulsating electric signal is pro duced.

6. In a method for operating a mass spectrom eter having a collectingmeans, including at least one ion collecting member, and having meansfor focusing a beam of ions corresponding to a selected mass upon-saidcollecting means to produce an electric signal, the improvement whichcomprises the steps of subjecting the focused ion beam to a transversemagnetic field of periodically changing polarity and of such strengththat the ion beam is periodically deflected off of said collectingmember, collecting a portion of the resulting deflected ion beam wherebya pulsating electric signal is produced, and amplifyin said pulsatingelectric signal.

7. In a method for operating a mass spectrometer having a collectingmeans, including a space pair of collecting members, and having meansfor focusing a beam of ions corresponding to a selected mass upon saidcollecting means to produce an electric signaLthe improvement whichcomprises the steps of subjecting the focused ion beam to a transversemagnetic field of periodically changing polarity and of such strengththat the ion beam is periodically deflected from one collecting member,to the other of said pair, collecting portions of the resultingdeflected ion beam whereby a pulsating electric signal is produced, andamplifying said pulsating electric signal.

ROBERT- WARREN LONG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,096,653 Soller Oct. 19, 19372,314,302 Ziebolz Mar. 16, 1943 2,331,189 Hipple Oct. 5, 1943 2,341,551Hoover, Jr Feb. 15, 1944 2,457,162 Langmuir Dec. 28, 19-13

